s/^(\t+)/' ' x (4 * length $1)/e;
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The number of spaces will be four times the number of tabs.

Or, without /e :

$s =~ s/(^|\G)\t/    /g;
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Replace a tab with 4 spaces if it's at the start of the string or after a previously replaced tab.

Update: The latter could be simplified to

$s =~ s/\G\t/    /g;
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as the "last match" is initially set to the beginning of the string.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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++

With the /m modifier, the first solution also works on multiline input: print $_ and print s/(^|\G)\t/____/mgr for "\tHello\t\n\t\tWorld\t!\n\t!\n";

Thank you very much! I was not aware of \G in perl, I will look into it.

what is the meaning of the \G there??

Anchor where to start for next match.

The \G anchor matches at the point where the previous//g match left off. \G allows us to easily do context-sensitive matching... perlretut

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1190410

use strict;
use warnings;

my $input = <<END;
|   |   |   |   |   |   |   |   !   !
first line
\t\tsecond line
\t\t\tthird line
\tfourth line
\t\tfifth line
\t\tline with\t\tin the middle
last line
END

open my $fh, '<', \$input or die "$! opening file";
while(<$fh>)
  {
  s/\G\t/    /g;
  print;
  }
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